NRG and Midwest Generation, the previous operator of the plants, have been under pressure from federal regulators for years to reduce coal-related emissions from the power plants.

The workforce at the Romeoville plant was reduced from 116 to 70 last year as one of the two power-generating units was shut down, NRG spokesman David Gaier said.

In Joliet, the power station is being switched to all natural gas, a process expected to be completed this year.

The Joliet and Romeoville stations were built in the 1950s by Commonwealth Edison as coal-fired power plants. They were later acquired by California-based Edison Mission Energy, which operated the stations under its Midwest Generation subsidiary.

NRG in 2014 acquired the local plants, as well as two others in Waukegan and Pekin that were operated by Midwest Generation. Parent company Edison Mission Energy was in bankruptcy at the time NRG acquired the plants.

New Jersey-based NRG also owns smaller power stations in Aurora and Rockford.

Gaier said some of the union employees in Joliet exercised bumping rights to take jobs at other NRG plants. The total number of Joliet employees leaving the company is 57.

The job reductions in Joliet include 18 management positions. Five managers are being transferred to other NRG plants, Gaier said.

In the layoff notice filed with the state of Illinois, NRG said layoffs will begin April 29 and end Oct. 7.